Kids in America usually enter a grade at age (grade + 5). Depends on when their birthday lands in some areas. For example, my kid was born in early September and the cutoff is Sept 1. His Kindergarten (grade 0) didn’t actually start until his birthday, so he entered at age 6.
Then the instance admin holds the private key and can still decrypt.
If you cared that much about privacy in DMs, we should have a “profile page”. Post a PGP public key there. Then you can send PGP encrypted messages to anyone who you have a public key for.